1. Field of the Invention
The present invention relates to secondary air suppliers for supplying secondary air to upstream of an emission purifying device of an internal combustion engine and, more particularly, to a failure diagnosis apparatus of the secondary air supplier capable of detecting an anomaly of a component thereof
2. Related Background Art
An example of the known emission control systems of internal combustion engine is an apparatus with a three way catalyst in the exhaust system to reduce CO, HC, and NOx components in exhaust gas and thereby clean up exhaust emissions. Furthermore, there is a known technique of connecting a secondary air supply path equipped with a switching valve, to an exhaust manifold, and pressure-feeding air (secondary air) into the exhaust manifold by means of an air pump to increase the oxygen content in exhaust, thereby effecting the cleanup based on promotion of oxidation of HC and CO in exhaust.
In the secondary air supplier of this type, where an anomaly occurs in such a component as the air pump or the switching valve, the cleanup efficiency of exhaust will drop, so as to worsen emissions. Therefore, it is necessary to promptly determine the anomaly. A known technology of detecting the anomaly of this kind is the one disclosed in Japanese Patent Application Laid-Open No. 9-125945. In this technology, a pressure sensor is placed on the secondary air supply path and, when a difference between a maximum and a minimum of pressure pulsation detected thereby is smaller than a predetermined value, it is determined that the secondary air supplier is abnormal, because of the absence of normal exhaust pulsation.
However, if the presence of exhaust pulsation is determined based on the condition that the difference between the maximum and minimum of pressure values is larger than the predetermined value as described above, there will arise the following problem. Namely, for example, in cases where there is a pressure pulsation caused by another source different from the exhaust pulsation, like noise on the values detected by the pressure sensor and where the aforementioned predetermined value for the determination is set small, there is a possibility of erroneously determining the pressure pulsation as the exhaust pulsation. Even if the predetermined value for the determination is set large in order to prevent it, surging of the air pump can also cause the pressure pulsation and in that case it might be erroneously determined as the exhaust pulsation.